Path MTU Discovery is a protocol to find the minimum maximum transmission unit (MTU) of links on a network path and adjust the MTU automatically. An MTU is the maximum size of an IP datagram transmittable in one data transfer. In the Ethernet® local area network (LAN) environment, for example, the maximum size of an Ethernet frame is 1518 bytes and the MTU is given as 1500 bytes by excluding the Ethernet header (14 bytes) and frame check sequence (FCS, 4 bytes).
For example, a case in which, as depicted in FIG. 1, data is transmitted from a terminal A to a terminal B is considered. According to the protocol, when a packet with an MTU of 1500 is transmitted (step (1)), a router C discards the packet because the MTU between the router C and a router D is 1400, and returns an error to the terminal A, which has transmitted the packet, by using an internet control message protocol (ICMP) packet (step (2)). The process in step (2) is performed under the condition that the “don't fragment” (DF) flag is turned on (that is, fragmentation is forbidden) in the packet. In the error message by ICMP, a usable MTU (1400 in this case) is included. The terminal A transmits a packet again based on the MTU of 1400 (step (3)). If the DF flag is not set, fragmentation is conducted at the router C, causing the throughput to decrease.
In a data center or the like, many virtual machines (VMs) are launched on many physical machines. As depicted in FIG. 2, there is a case in which a virtual machine launched on a physical machine A (VMA) in a data center DC1 communicates with a virtual machine launched on a physical machine B (VMB) in another data center DC2, which is connected to the data center DC1 via a network such as a layer 3 (L3) network. In this case, communication is performed sometimes by constructing a virtual L2 network on the network such as an L3 network. This virtual L2 network is called an L2 tunnel, on which communication is performed by encapsulating an L2 packet with an L3 packet. When an L2 network is constructed, a tunnel endpoint of the L2 network is created on the host OS, which is a base operating system (OS) for a virtual machine environment on a physical machine.
When communication is performed in such an environment, the MTU of an L2 packet transmitted on the L2 network is to be determined in accordance with the MTU of an L3 packet transmitted on the L3 network. However, there is a case in which, when a change takes place in the L3 network configuration, not only the MTU of an L3 packet but also the MTU of an L2 packet changes. If a migration, launch, stop, or the like of a virtual machine takes place, there is a possibility that it causes construction or removal of an L2 tunnel and leads to a change in the MTU of L2 packets in use. No related art technology that may cope with such a problem has been developed. If a packet is generated with an inappropriate MTU, there is a possibility that fragmentation takes place on the communication path, causing a decrease in the throughput.
Japanese Laid-open Patent Publication No. 2011-186805 and Japanese National Publication of International Patent Application No. 2004-535713 are examples of the related art.